Schizophrenia (SCZ) affects approximately 1% of the world?s population and is characterized by a constellation of symptoms that include hallucinations and delusions (positive symptoms); antisocial behavior and blunted emotions (negative symptoms); and deficits in working memory, executive function, and learning and memory (cognitive symptoms). Antipsychotics primarily acting through dopamine receptors (Drd2s) effectively alleviate positive symptoms, to a lesser extent negative symptoms, and are mostly ineffective for cognitive symptoms. This clinical picture implicates multiple neural circuits and mechanisms in different SCZ symptom categories. Because the etiology of SCZ is unknown and valid SCZ mouse models are not available, we focus on 22q11.2 deletion syndrome (22q11DS), the most common microdeletion syndrome in humans, which increases the risk of SCZ 30 fold. Psychotic symptoms are clinically indistinguishable in patients with SCZ with or without 22q11DS and usually appear during late adolescence or early adulthood. Mouse models of 22q11DS (22q11DS mice) have been constructed and validated. Using these mutant mice, we and others have identified cellular and molecular mechanisms underlying the cognitive and negative symptoms of 22q11DS. During the previous funding period, we identified a specific disruption of synaptic transmission in thalamocortical (TC) projections between the auditory thalamus and auditory cortex (ACx) in 22q11DS mice. Abnormal activity in these brain regions in humans is associated with auditory hallucinations. Disruption of TC projections occurs in mice at 3.5 months, which corresponds to late adolescence/young adulthood in humans and is rescued by antipsychotics and specific Drd2 inhibitors. Our studies revealed that the TC deficit is caused by reduced glutamate release from thalamic afferents, resulting from the haploinsufficiency of a 22q11DS gene Dgcr8, which mediates microRNA (miR) biosynthesis. Dgcr8 haploinsufficiency leads to depletion of miR-338- 3p, which in turn, elevates Drd2 levels in thalamic relay neurons. Elevated Drd2 decreases glutamate release in thalamic neurons. The expression of miR-338-3p was enriched in the thalamus and declined with age, which may underlie thalamus specificity and the mechanism of late onset of TC disruption. Although the TC mechanism appears to satisfy requirements for mediating positive symptoms, how it affects network activity in the ACx and auditory thalamus is unclear. In this competitive renewal application, we propose to analyze abnormal spontaneous activity in individual neurons of the ACx and auditory thalamus in behaving mice. For the ACx, we will use 2-photon imaging through a cranial window, and for the auditory thalamus, we will use a head-attached miniscope (1-photon imaging) or 2-photon imaging through graded index lenses. We also propose to investigate the mechanisms underlying age-dependent changes in the expression of miR-338-3p. We believe that these experiments will elucidate new mechanisms of the most enigmatic symptoms of SCZ and provide a framework for the future development of specific therapeutic interventions to alleviate positive symptoms in patients with this catastrophic disease.